This invention relates to processes for irradiating and grinding PTFE to reduce the molecular weight and render the material grindable to a fine particle powder, which can be used as a dry lubricant in paint and ink.
An early description of irradiation of PTFE is contained in U.S. Pat. No. 3,766,031 to Dillon. According to this patent material was placed in trays and subjected to irradiation.
U.S. Pat. Nos. 4,748,005 and 4,777,192, which are owned by the assignee of the present invention, discloses commercial batch processing of PTFE wherein, the material is placed in a ribbon blender and wherein electron beam radiation is directed into a portion of the blender while the material is agitated by the blender. The blender is provided with cooling either by a water jacket or by direct injection of water into the vessel. The ribbon blender is operated to continuously move the material into and out of the irradiation zone, thereby to achieve uniform irradiation.
U.S. Pat. No. 5,149,727 discloses a processing vessel which includes two vessel portions, each having a paddle blender agitator. Air is injected into the process material to promote cooling, coupled with simultaneous irradiation and grinding. An air classifier is used to draw airborne fine particle PTFE from the vessel for recovery. U.S. Pat. No. 5,296,113 discloses a similar system wherein the agitation is provided by use of injected high pressure air, and wherein material in continuously fed to the processing vessel and extracted by an air classifier to achieve a continuous process.
It is an object of the present invention to provide an improved method and apparatus for processing PTFE.
According to the invention an improvement is provided in an apparatus including a processing vessel for processing PTFE by irradiation and grinding. According to the invention the vessel includes transversely horizontally adjacent first and second vessel portions having corresponding first and second agitators arranged for rotation about a longitudinal axis. The inner floor of the vessel has a step between the first and second vessel portions, so that the inner floor of the first vessel portion is lower than the inner floor of the second vessel portion. A plurality of air nozzles are provided for injecting air toward the first vessel portion, the nozzles being mounted on a substantially vertical portion of the step.
In a preferred arrangement the first agitator rotates in a direction against the direction of air from the air nozzles in the lower part of the first vessel portion. Preferably the second agitator is arranged for rotation about its longitudinal axis in the same rotational direction as the first agitator, so that the agitators move in opposite directions relative to each other at the center of the vessel. The transverse outer wall of the second vessel portion can be cylindrical and have an interior space less than one inch from the periphery of the second agitator. An electron beam source can be arranged for irradiating PTFE material at the top of the second vessel portion.
In accordance with the invention, there is provided apparatus for preparing scrap PTFE for processing and for supplying the PTFE to a processing vessel. The apparatus includes a chopper for cutting scrap PTFE into granular chips, a blender for receiving the chips from the chopper and for agitating the chips to promote flow, and a conveyor for receiving the chips from an outlet of the blender and for carrying the chips toward the processing vessel.
In accordance with the invention, there is provided an improved process for irradiating and grinding PTFE in a processing vessel. According to the improved process, pressurized air at a temperature of 400 to 700xc2x0 F. is injected into the vessel for grinding the PTFE, and the processing vessel open to admit ambient air drawn into the vessel in a volume 2 to 5 times the volume of the pressurized air.
For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.